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FILE - In this Aug. 2, 2007 file photo, Brendan Dassey is escorted into court for his sentencing in Manitowoc, Wis. The Netflix documentary series “Making a Murderer” tells the story of a Wisconsin man wrongly convicted of sexual assault only to be accused, along with his nephew, of killing a photographer two years after being released. Steven Avery and his then 17-year-old nephew Dassey were accused of killing Teresa Halbach, a photographer who visited the Avery family salvage yard to take photos of a minivan on Halloween and was never seen alive again.

Wisconsin's attorney general asked a federal appeals court Wednesday to let stand the conviction of a man found guilty in a case profiled in the popular "Making a Murderer" series on Netflix.

A federal magistrate judge ruled in August that investigators tricked Brendan Dassey into confessing he helped his uncle, Steven Avery, rape, kill and mutilate photographer Teresa Halbach in 2005. Dassey, who turned 27 Wednesday, was 16 at the time. The magistrate ordered that Dassey be freed unless prosecutors appealed or decided to retry him.

In a brief filed with the 7th U.S. Circuit Court of Appeals, Attorney General Brad Schimel urged the appeals court to reject Dassey's claim that his confession was coerced.

"Substantial police coercion" is required for any confession to be ruled involuntary, Schmel said. And he said the Wisconsin Court of Appeals was right to affirm in 2013 that Dassey's confession was voluntary.

Magistrate Judge William Duffin held that investigators made specific promises of leniency to Dassey and that no "fair-minded jurists could disagree." He cited one investigator's comment early in the interview that "you don't have to worry about things," plus repeated comments like "it's OK" and that they already knew what happened.

But the magistrate's ruling "ignores both the facts and the law," the attorney general said. Investigators didn't promise leniency, he said, and specifically told Dassey they couldn't make any promises.

The teenager willingly spoke with investigators and was properly informed of his rights, Schimel said. The interview took a few hours in the middle of the day, while Dassey sat on a couch and drank a soda, the investigators spoke in normal tones, and did not threaten him or make false promises, he said. And Dassey confessed to most of the important details within an hour, in response to open-ended questions, he added.

"The state courts' conclusion that Dassey's confession was voluntary is not only reasonable; it is entirely correct. Accordingly, Dassey is not entitled to relief," the attorney general said.

Halbach was killed on Halloween 2005, after she visited the Avery family's salvage yard in Manitowoc County. Investigators allege Avery lured her there by asking her to take photos of a minivan. Dassey was sentenced to life in prison in 2007. Court documents describe him as a slow learner who had poor grades and has difficulty understanding language and speaking. Avery was convicted in a separate trial and was also sentenced to life in prison. He's pursuing his own appeal.

Their cases gained national attention after Netflix aired "Making a Murderer" last year. The series spawned widespread conjecture about the pair's innocence. Authorities who worked on the cases said the series was biased, but it generated a myriad of calls from the public to free both men.